The invention concerns a shoe press belt for use in shoe presses of a paper machine, having a support and a liquid-impermeable belt layer which has an inner layer and an outer layer adjacent thereto, the outer layer having a porous structure and the porous structure being formed exclusively from cavities open toward the outer side.
A shoe press belt of this kind is evident from FIGS. 6 and 7 of U.S. Pat. No. 4,701,368. It has a liquid-impermeable belt layer that is constructed in two layers, with a liquid-impermeable inner layer and an outer layer adjacent thereto. In the one exemplary embodiment an additional support is present in the form of a fabric, while in the other exemplary embodiment the inner layer also simultaneously has a support function and thus forms the support. The outer layer, which is intended for direct contact against the paper web, can be made of a closed-pore or open-pore foam material; in the latter case, dewatering of the paper web is accomplished by way of the outer layer, and a press felt can thus be dispensed with.
The known shoe press belt has the disadvantage that the outer layer becomes practically completely compressed under the high pressure of the shoe press, since it consists only of the thin cell walls of the foam. The dewatering that is desirable at least with the open-pore version therefore occurs insufficiently or not at all.
U.S. Pat. No. 4,552,620 discloses a shoe press belt that comprises a woven support and a belt layer, applied on one or both sides, that is equipped throughout with a limited number of non-communicating pores. The pores have a diameter of 0.019 to 0.185 mm, and are said to produce a stone-like texture on the outer side provided for contact against the paper web. This texture is said to facilitate separation of the paper web from the shoe press belt after passing through the shoe press.
The shoe press belt described above has the disadvantage that the belt layer is very elastic because of the pores distributed over the entire cross section, and that it is therefore greatly compressed in the shoe press nip, with the consequence that the pores are also compressed. The pores are therefore not provided at all for the purpose of improving dewatering of the paper web, and also cannot do so.
It is the object of the invention to configure a shoe press belt of the kind cited initially in such a way that it is substantially more resistant to compression in the press nip and accordingly guarantees effective dewatering of the paper web.
This object is achieved, according to the present invention, in that the outer layer is made of an unfoamed material, i.e. preferably of a plastic material in which, however, only pores that are open toward the outer side are present. Otherwise the belt layer is homogeneous, and can therefore be adapted in accordance with particular requirements in terms of hardness, modulus of elasticity, etc. It has been found that with a shoe press belt constructed in this fashion, effective dewatering of the paper web (optionally assisted by a co-running press felt) is obtained.
The inner layer is advantageously of liquid-impermeable configuration and can be of longitudinally elastic and/or compressively elastic configuration. Preferably the specific modulus of the support should be xe2x89xa6500 cN/tex. Materials such as PBT, PES, PA-6, PA-6,6, PA-6,10, PA-6,12, PA-11, PA-12, and PTT are suitable in particular for the inner layer; these materials can also be combined with one another.
As in the case of all belts for a paper machine, the support ensures the structural strength of the shoe press belt. For this purpose the support can be constructed of threads, for example in the form of a woven fabric, knitted fabric, or thread layer. Also suitable, however, are fiber batts of appropriately solid configuration, for example in impregnated or compressed form; if possible, these should possess a uniform thickness. On the side on which the coating is applied, the surface should be smooth, for example polished. In order to create a permanent join between support and coating, it is advantageous if the support is at least partially embedded into the coating. Complete embedding is also possible.
Natural rubber or an elastomer are suitable as the material for the inner layer. Silicone elastomer, polyester elastomer, and polyurethane are particularly suitable. The hardness of the inner layer should preferably be between 80 and 95 Shore A.
Inorganic filler particles, for example TiO2 or clay, can additionally be incorporated into the inner layer in order to influence its hardness. It is advantageous in terms of the functionality of the inner layer if it has a thickness tolerance of max. 100 xcexcm. To achieve such a tolerance, it can be appropriately machined down and polished before application of the outer layer.
Polyurethane and/or silicone elastomer and/or polyester elastomer is preferably suitable as the material for the outer layer. When these or other plastic materials are used, the cavities can be produced, in a manner known per se, by the fact that soluble particles are scattered onto and embedded into them, and are dissolved out with a solvent to which the outer layer is resistant (cf. EP-A 0 786 550). Water-soluble particles in the form of salts such as NaCl, KCl, and/or CaCO3 are particularly suitable for this purpose. The particles should have a diameter of 10 xcexcm to 1500 xcexcm, preferably between 400 xcexcm and 1000 xcexcm, in a random distribution, in order to generate cavities of appropriate size.
In order to improve the wear resistance of the outer layer, it is proposed to equip it on its surface with a layer of nanoparticles. These particles, used heretofore in chemistry as pigments for color effects, cosmetics, and data storage layers, whose particle sizes are in the nanometer range, can effectively protect the outer layer from wear, in particular if the nanoparticles are made, for example, of SiO2 or metals and form an almost continuous layer. The nanoparticles can be applied as a sol, the solvent (usually alcohol) then being evaporated. The nanoparticles can be equipped locally with fluorocarbon chains in order to give surface regions of the outer layer a hydrophobic character, and thereby to facilitate separation of the paper web from the shoe press belt.
A further alternative for producing the outer layer is to use an electron beam-cured prepolymer emulsion. Particularly suitable for this purpose are silicones or polyurethanes that are emulsified in a water-surfactant mixture which is evaporated upon electron beam curing.
Provision is also made according to the present invention for the outer layer to comprise, on the outer side, materials which form regions of differing hydrophilicity and hydrophobicity. Both are intended to facilitate separation of the paper web from the shoe press belt; the regions and the differences in terms of hydrophilicity and hydrophobicity are to be arranged and configured so that sufficient adhesion of the paper web is still ensured in the region where the press felt lifts off.
The shoe press belt advantageously has a hardness of between 80 Shore A and 95 Shore A, and a thickness tolerance of xc2x150 xcexcm.
Provision is also made according to the present invention for a further layer, which is harder than the outer layer, to be provided between the outer layer and inner layer.
Lastly, it is proposed according to the present invention that the complete shoe press belt have a thickness tolerance of xc2x1100 xcexcm.